The present invention relates to thermochromic indicators for use in cookware, and in particular to a lid for a cookware vessel.
Thermochromic inks and paints of various compositions are well known for their ability to reversibly change color, or another optical property, upon reaching a predetermined temperature. Such paints or coatings have been applied to cookware vessels to indicate that the pan has reached a suitable temperature for adding foodstuffs, or to indicate that the contents have reached a desired temperature for another phase of cooking, as well as to warn the user and others in the kitchen that the cookware article is hot and should not be touched with the bare hand. For example, U.S. Pat. No. 3,701,344 to Walls et al. (issued Oct. 31, 1972), which is incorporated herein by reference, discloses a cooking apparatus with a knob having an indicator, which by its change in color can enable a cook to manipulate the heat in order to obtain the best results in using waterless cookware. The color changing indicator is a thermochromic paint that embedded within a knob style handle on the lid of the cookware article. A thermally conductive heat sink within the knob style handle conducts heat from the lid to the thermochromic paint, which is visible through a clear window at the center of the knob. This initial color change within the knob is intended to indicate when heat should not longer be applied to the waterless cookware article. Then, the return of the knob to the original temperature on cooling is intended to indicate when the food is cooked by the residual steam retained in the waterless cookware vessel.
U.S. Pat. No. 4,805,188 to Parker (issued Feb. 14, 1989), which is incorporated herein by reference, discloses a time-temperature indicator, particularly adapted for use with closed sterilizing or cooking vessels to indicate at what temperature and for how long material contained within the vessel has been heated or is cooking. The indicator deploys a thermochromic coating on a thermally conducting cylinder or similar shaped object having one end in thermal contact with the lid of the cooking vessel. This thermally conductive cylinder is surrounded by a transparent thermally insulating material such that as heat is conducted up the cylinder from the lid an ever increasing area of the thermochromic coating thereon will change color in proportion to the time that the lid was at or above the thermochromic transition temperature. The thermochromic coating will be visible through the surrounding transparent thermally insulating material. The indicator may be constructed as a knob on the lid of a pressure cooker or sterilizer or may be attached to replace an existing knob. While the device can be constructed to provide readings with the full 360° angle around the knob or indicator, the user is still require to focus their attention on a relatively small area of the cookware article.
In addition, U.S. Pat. No. 5,720,555 to Elele (issued Feb. 24, 1998), which is incorporated herein by reference, discloses a temperature indicating container and lid apparatus. According to the disclosure of the '555 patent, the temperature indicating container apparatus includes an inner container portion made from substantially heat insulating material. A thermochromic-substance-containing portion is juxtaposed against an outside surface of the inner container portion, such that color changes of the thermochromic-substance-containing portion can be seen from outside the inner container portion. The thermochromic-substance-containing portion may be in a form of a jacket around the outside surface of the inner container portion. The jacket may include a plurality of windows. An outermost container portion may be located outside both the thermochromic-substance-containing jacket and the inner container portion. The outermost container portion is light transmissive such that color changes of the thermochromic-substance-containing jacket can be seen from outside the outermost container portion. A lid assembly includes a lid portion for covering the inner container portion and the thermochromic-substance-containing portion. The patent discloses that the lid portion may include thermochromic substances. As the container itself is thermally insulating, it is not suitable for use as an article of cookware that is externally heated. Further, the aforementioned lid and lid assembly are generally specific to either a particular pot, cooking technique or thermally insulating container.
FR1388029 teaches the attachment of a thermochromic indicator spot on the inner cooking surface of a frypan. The spot changes color at a predetermined temperature to indicate to the cook or user than the pan is hot enough and that food stuffs to be cooked should be inserted. U.S. Pat. No. 6,551,693 to Buffard et al. (issued Apr. 22, 2003), which is incorporated herein by reference, similarly discloses a cooking container, having an inside surface coating including a decoration which changes color as a function of temperature. The decoration is based on a thermostable resin and a chemical substance which changes color as a function of temperature. This decoration is applied over a fluorocarbon resin coating on the inside of the container. However, the color changing devices of FR1388029 and the '693 patent require the cook or user to stand over the cooking vessel to inspect the thermochromic spot or decoration. Such spot or decoration does not indicate when the food is cooked, and in fact would be obscured when the foodstuffs are added to the pan or cookware article.